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the NE-SW to ENE-WSW regional S Hmax an M 2.9 on 11 August. The aftershock toward the SW. These movements are
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(Snee and Zoback, 2020), favoring them as sequence was mainly recorded with a real- consistent with the determined focal mech-
potentially seismogenic. However, these time broadband seismic array installed by anism and InSAR analysis.
WNW-trending structures are not included CERI 48 h after the main shock. Most Along the central portion of the surface
in the USGS Quaternary Faults or U.S. aftershocks were M <1.5, shallower than rupture, several 250 MHz ground-penetrat-
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Seismic Source Characterization for Nuclear 3 km, and distributed across a 40-km ing radar (GPR) profiles were acquired per-
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Facilities databases (Crone and Wheeler, elliptical area with its major axis trending pendicular to the surface rupture. The GPR
2000; Machette et al., 2004; U.S. Nuclear NW to WNW (USGS Catalog August– profiles consistently show sub-horizontal
Energy Regulatory Commission, 2012). February 2021; Fig. 1A). The plotted after- reflectors in the upper ~4 m crossed by a
shock hypocenters projected onto a cross few 20–30° south-dipping reflectors. While
THE SPARTA EARTHQUAKE section normal to the rupture suggest that this is the expected co-seismic rupture
The surface rupturing M 5.1 Sparta the earthquake sequence is associated with geometry, one dipping reflector projects to
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earthquake occurred on a WNW-striking a SW- dipping structure (Fig. 1C). the surface a few meters to the north of the
previously unknown structure, now named surface rupture (Fig. 3G).
the Little River fault (Hill et al., 2020). The RECOGNITION OF THE SURFACE
main event generated peak ground accelera- DEFORMATION GEOLOGICAL ANALYSIS OF
tions of at least 0.2 g with an MMI of VI–VII The recognition and mapping of a surface SURFACE DEFORMATION
in Sparta (USGS, 2020a). Roads, utility rupture trending ~N110° began on the day No major ground cracking was evident
lines, and masonry structures were dam- of the earthquake and continued for several near the epicenter. The majority of the sur-
aged, including the collapse of walls and months. The collection of uncrewed aerial face rupture is located to the south and south-
chimneys and the cracking and shifting of systems (UAS) imagery and processing of east of Sparta, and north of U.S. Route 21,
foundations, causing North Carolina’s gov- digital terrain models aided field mapping, stretching for ~2 km and across a generally
ernor to declare a local state of emergency highlighting minor topographic changes <25-m-wide zone. The most prominent fea-
and the North Carolina General Assembly to along the surface rupture and identifying tures and primary evidence of surface rup-
provide U.S.$24M for earthquake recovery small-scale deformation features (Figs. ture are straight ten to hundreds of meters
(Office of State Budget and Management, 2B–2F). In addition, the co-seismic scarps long, ~N110°-trending occasionally en éche-
2020, pers. comm.). Fortunately, there were were surveyed with a real-time kinematic lon ground ruptures. Along the rupture, a co-
no casualties or significant damage to major global positioning system to measure seismic topographic step, formed by reverse
infrastructures. displacements. scarps and folding/flexure of the topography,
The mainshock hypocenter and focal A preliminary interferometric synthetic has an average height of 8–10 cm and a maxi-
mechanism have been estimated indepen- aperture radar (InSAR) analysis was con- mum of ~25 cm. The southern block is con-
dently by several groups. Horton et al. ducted using ascending Sentinel-1A imagery sistently uplifted. The surface rupture has
(2021) at the Center for Earthquake acquired a day before the Sparta earthquake four sections (A to D in Fig. 2A):
Research and Information, University of (8 August) and Sentinel 1B imagery acquired 1. Greenway Drive industrial park (sec-
Memphis (CERI), calculated a first-motion six days later (14 August). Despite areas of tion A): South of downtown Sparta sev-
focal mechanism consistent with a N108°- poor coherence, the unwrapped interfero- eral ground fissures (crossing U.S. Route
striking 60° SW-dipping nodal plane, with gram delineates an area of deformation of 21) and small scarps align in strike for a
a 24° rake (Fig. 1B) and a best-fitting cen- ~20 km (Fig. 1A). An irregular contact distance of ~300 m as they cross the
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troid depth of 0.6 km based on modeling between a positive and negative line-of-sight industrial park at Greenway Drive. The
regional waveforms and an epicenter loca- (LOS) trends ~N125° for ~3-km, roughly structures trend N100–110°, and some
tion (36.488° N, 81.106° W) using a grid coincident with the mapped co-seismic sur- exhibit a subtle right-stepping en échelon
search procedure. The Saint Louis face rupture. The southern block has a nega- pattern. The scarps are single or multiple
University Earthquake Center calculated a tive LOS (movement away from the ENE- minor steps, building to a ~20 cm high
centroid depth of 1 km with a N115°, 50° looking satellite), while the northern block maximum (Figs. 2B–2D). Folding associ-
SW nodal plane and rake of 40° based on has a positive LOS (movement toward the ated with the uplift caused extension at
best-fit modeling of regional waveforms satellite). These patterns are consistent with the top of the hanging wall with oblique
(SLUEC, 2020). Analysis by SLUEC (2020) left-lateral reverse motion occurring along fissures and cracks. Evidence for lateral
and Horton et al. (2021) yield similar results the SW-dipping nodal plane identified in the displacement is minor, and no marker
consistent with a shallow (<1 km) left- focal mechanism. was laterally displaced across the rupture
lateral reverse rupture on a SW-dipping In the hanging wall, located ~300 m and trace. Several buildings were moderately
plane. These results, however, differ from 600 m from the main surface rupture, the damaged in the industrial park, particu-
the preferred USGS solution, which places Alleghany 13 and Funeral geodetic monu- larly those on the surface rupture. Many
the event deeper (4.1 ± 1.8 km), with a nodal ments surveyed by the North Carolina secondary ground fissures were induced
plane striking N176°, 48° W and a rake of Geodetic Survey in September–November by ground shaking to the south and west
136° (USGS, 2020b). The earthquake 2020 moved 19.7 cm to the ESE and 5.7 cm of the industrial park (Fig. 2A).
sequence started with eight foreshocks with to the ENE, respectively, and Alleghany 13 2. Little River valley (section B): The
M 1.8–2.6 during the 24 h before the main- rose 15 cm (Fig. 2A). The geodetic monu- rupture crosses the Little River valley
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shock, followed by at least 300 aftershocks ment Alleghany 15, located ~600 m north of for ~500 m along a steep and densely
over the next six months, the largest being the main surface rupture shifted 7.8 cm vegetated slope that hampers features

6 GSA TODAY | March-April 2022